I'm making some ivy wrap around some iron work. I have some copper ivy leaves that I want to join to mild steel vines. Is there any way to do this other than provide a barrier between the two metals? Is there a way to weld them together?

Tig weld with silicon bronze, silver solder or torch weld w/silicon bronze.

I'd braze it, do it all the time just keep most of the heat on the steel.

Braze with what kind of rod?

Braze with what kind of rod?

Just your average flux coated bronze brazing rod. Im never to specific at the welding store I tell them 5 pounds of brazing rods and I get what they give me and Ive never had a problem.

Same for me. Brass melts easier than copper so it will work ok. I don't even use brazing rod much of the time. I just use spelter ( bits of old brass ferrules cut up) and flux with borax. A MAPP gas torch will do it on smaller work or you can do it in a forge. Clean the joints well and wire the pieces together before taking a heat.

Is this for an outside item that will get wet? Mixed metal often have issues with galvanic corrosion in such cases that you should be aware of. Inside usually not.

Yes, it is outside. It will be on the shaded side of the house, not in direct sunlight, if that matters.

Coat with a good clear polymer finish. That will keep the joints dry and the metals from rust and/or corrosion. Or for a more traditional finish, bees wax.